This invention relates to the improvement of a crystal element, more particularly to a crystal element utilizing a DT-cut quartz crystal for providing vibrations of the contour mode.
It is well known in the art to use a DT-cut crystal element for producing a fundamental wave vibration in a relatively low frequency band of from about 100 to 300 KHz and to use a CT-cut crystal element for producing a fundamental wave vibration in a relatively high frequency band of from about 300 to 600 KHz. The frequency constants produced by said two types of crystal elements generally have the following values. More particularly, with a DT-cut crystal element made of a square quartz plate, a frequency constant of 2080 KHz.sup.. mm is produced which corresponds to the product of the natural frequency and the length (in mm) of one side of the square. In the case of a circular disc, the frequency constant is expressed by the product of the natural frequency and the diameter (in mm) of the disc, that is 2480 KHz.sup.. mm. In the case of the CT-cut, where a square crystal element is used, a frequency constant of 3090 KHz.sup.. mm is obtained, whereas (when a circular disc crystal element is used) a frequency constant of 3780 KHz is obtained. In this manner, the CT-cut crystal element is superior to the DT-cut crystal element in frequency characteristics but the DT-cut crystal element has better frequency-temperature characteristic. For this reason it will be very advantageous if the operating frequency range of the DT-cut crystal element can be widened to that of the CT-cut crystal element by modifying the orientation of the cut.
The frequency of a crystal element for producing a contour mode vibration is generally determined by the orientation of the cutting angle with respect to the axis of crystallization and the contour dimension of the crystal element, and the frequency is not related to the thickness of the crystal element, so that the orientation of the cutting angle is firstly determined to be the CT cut or DT cut thus determining the frequency constant, and then the contour dimension corresponding to the desired frequency is determined by utilizing the frequency constant. Therefore, the thickness of the crystal element has been made to be independent of the frequency. For this reason, convexed element was utilized for crystal elements operating at a thickness shear mode vibration alone and not for crystal elements operating with a contour mode vibration.